Diabetes and obesity are increasing health problems globally and are associated with various other diseases, particularly cardiovascular diseases (CVD), obstructive sleep apnea, stroke, peripheral artery disease, microvascular complications and osteoarthritis. There are 246 million people worldwide with diabetes, and by 2025 it is estimated that 380 million will have diabetes. Many have additional cardiovascular risk factors including high/aberrant LDL and triglycerides and low HDL. Cardiovascular diseases account for about 50% of the mortality in people with diabetes, and the morbidity and mortality rates relating to obesity and diabetes underscore the medical need for efficacious treatment options.
Incretins are gastrointestinal hormones that regulate blood glucose by enhancing glucose-stimulated insulin secretion (Drucker, D J and Nauck, M A, Lancet 368: 1696-705 (2006)). To date there are two known incretins: glucagon-like peptide-1 (GLP-1), and glucose-dependent insulinotropic polypeptide (GIP). The incretin GLP-1 is derived from the pre-proglucagon gene. Pre-proglucagon is a 158-amino acid precursor polypeptide that is processed in different tissues to form a number of different proglucagon-derived peptides, including glucagon, GLP-1, glucagon-like peptide-2 (GLP-2) and oxyntomodulin (OXM). Glucagon is a 29-amino acid peptide that corresponds to amino acids 33 through 61 of pre-proglucagon, while GLP-1 is produced as a 37-amino acid peptide that corresponds to amino acids 72 through 108 of pre-proglucagon. GIP is a 42-amino acid peptide derived by proteolytic processing from a 133-amino acid precursor, pre-pro-GIP. All the peptides are involved in a wide variety of physiological functions, including glucose homeostasis, insulin secretion, gastric emptying, and intestinal growth, as well as the regulation of food intake.
The discovery of the incretins has led to the development of two new classes of drugs for the treatment of diabetes mellitus. Thus, injectable GLP-1 receptor agonists, and small molecule compounds (oral DPP-4 inhibitors) that inhibit enzymatic inactivation of both endogenous GLP-1 and GIP, are now on the market (GLP-1 receptor agonists: Byetta™, Bydureon™ and Victoza™ and DPP-4 inhibitors: Januvia™, Galvus™, Onglyza™ and Trajenta™). Apart from the acute effects of GLP-1 and GIP on insulin secretion, the incretins have some long-term effects. Evidence from several laboratories shows that GLP-1 receptor agonists protect pancreatic β-cells by inhibiting apoptosis and enhancing proliferation. For instance, a study by Farilla et al. showed that GLP-1 has anti-apoptotic effects in human islets (Farilla, L, Endocrinology 144: 5149-58 (2003)). Such effects have not been reported for GIP until recently. Weidenmaier et al. reported that a DPP-4 resistant GIP analogue had anti-apoptotic effects (Weidenmaier, S D, PLOS One 5(3): e9590 (2010)). Interestingly, in mouse models of diabetes and obesity, the combination of the GLP-1 receptor agonist Liraglutide and GIP showed superior glucose-lowering and insulinotropic effects compared to treatment with Liraglutide and GIP alone (Gault, V A, Clinical Science 121: 107-117 (2011)).
Chronic treatment with GLP-1 receptor agonists causes significant weight loss in diabetic humans. Interestingly, extended use of DPP-4 inhibitors in similar patients does not consistently change body weight. Evidence suggests (Matthias Tschöp oral presentation at ADA (American Diabetes Association), 2011) that body weight loss associated with GLP-1 agonist treatment is enhanced when GLP-1 and GIP are co-administered. In rodents, co-administration of GLP-1 and GIP results in greater body weight loss than GLP-1 treatment alone. Thus, in addition to improving blood glucose control, GIP may also enhance GLP-1-mediated body weight loss.